|MadSci Network: Engineering|
Hi Leo: In concept, we need to build a 'Low-Pass filter'. This filter allows the low frequency audio tones to get through, but stops the higher frequencies to get through. There are many ways to build filters. The general catagorization may fall into two catagories of 'passive filters' and 'active filters'. A simple capacitor placed between the line and ground would be an example of a passive filter. It lets high frequencies pass through it to ground, effectively killing the high frequencies, and letting only the low frequency or DC components to remain on the line. I chose an active filter design as it does the above, but much better. I made some assumptions as follows. 1) I assumed the audio tone will be from an 'audio out' source, such as a tape recorder output, which is somewhere around 1Vpp. 2) I assumed that such things as phase and amplitude distortion are really not an issue here. 3) I assumed that the light may be larger than a flashlight type bulb. 4) I assumed that this is just to experiment with, to have fun, at a party perhaps lighting up on deep base tones. Furthermore, that it will not be used in any real application. Having said all that, here is the circuit with ideal values calculated: The circuit pictured here had the following response when I simulated it using PSPICE: But here is the catch - in real life, we can't get values accurate enough with decimal points. Heck, sometimes we are lucky if we can get close at all. So I went to the parts room, and I picked out the closest values that are commercially available, and proceeded to build the circuit shown below: The simulation with the actual values matched up with what I actually observed in the lab. My lab prototype stopped at about 144Hz. Not knowing what you intend to use this for, I would venture to guess that it will probably do just fine for your application. If, however, you need more accuracy, try to get as close to the ideal values as you can. Here is the response you will see if you use the commercially available values: I added a transistor to drive a relay (labeled L2- ignore the 10uH) in the schematic. The relay can be used to drive a light of your choice. The relay I used was a generic 5volt relay (Radio Shack). The potentiometer should provide for some trim-ability on when the relay kicks on and off. The supply voltage can vary, somewhat, between 5-12 volts. Just make sure the op-amps have a high enough voltage rating. Yes, you may substitute other op-amps if you choose. I used a National Semiconductor LM324, which has four amps on it, only two of which were needed for the filter. Watch out to not over drive the input. Don't connect the speaker output to this circuit. More protection circuitry is needed if the circuit is being driven off of a speaker port. To give you that protection circuit, I would need to know exactly what this circuit is being attached to. Good luck, and let me know how this turns out. Abtin Spantman ASPANTMAN@EXECPC.COM
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